Quadrature hybrid circuits are widely used as power divider and/or combiner circuits for power dividing or power combining of high frequency signals in radio frequency bands. FIG. 23 shows a configuration of a branch-line type quadrature hybrid circuit (hereinafter referred to as quadrature hybrid circuit). Four transmission lines 180, 181, 182, 183 are interconnected in a ring, and the four junction points of said transmission lines serve as I/O terminals for high frequency signals.
Transmission line 180 is connected to terminal 1 (hereinafter referred to as port 1) on one side, and to terminal 2 (hereinafter referred to as port 2) on the other side. Transmission line 181 is connected to port 2 on one side, and to terminal 3 (hereinafter referred to as port 3) on the other side. Transmission line 182 is connected to port 3 on one side, and to terminal 4 (hereinafter referred to as port 4) on the other side. Transmission line 183 is connected between port 4 and port 1.
Transmission lines 180 and 182, and transmission lines 181 and 183, which are faced each other, are respectively configured with identical characteristic impedances. The coupling factor between Port 1 and Port 3 can be changed according to the ratio of the characteristic impedance of transmission lines 180 and 181.
For example, let us assume that an identical load (impedance Z0) is connected to each of ports 2, 3, and 4, a signal source 184 with impedance Z0 is connected to port 1, and a high frequency signal is input into port 1. If, at this time, the characteristic impedance of transmission line 181 is Zb, and the characteristic impedance of transmission line 180 is Za=Zb/√{square root over (2)}, half of the power of the high frequency signal input into port 1 is output to port 3. The remaining half of the power is output to port 2, and the phase difference between the high frequency signals of port 2 and port 3 is 90 degrees. Attenuation to half of original signal power, expressed in decibels, is −3 dB. Therefore, such a circuit is referred to as a quadrature hybrid circuit with a coupling factor of 3 dB. Such a quadrature hybrid circuit is described on p. 185 of Microwave Solid State Circuit Design, Wiley-Interscience, John Wiley & Sons, Inc. (hereinafter referred to as non-patent document 1) as a quadrature hybrid, with the matching condition and the coupling factor leaded as equations (1) and (2).Matching condition: Y02=Ya2−Yb2  (1)Coupling factor: C=20 log10 Ya/Yb  (2)
In the above equations, Y0 is the admittance expression for Z0. Likewise, Ya and Yb are the admittance expressions for Za and Zb, respectively. As the characteristic impedance Za of transmission line 180 is Za=Zb/√{square root over (2)}, the admittance Ya=√{square root over (2)}Yb. Therefore, the coupling factor C is −3 dB.
By setting the ratio of admittance values as shown in equation (2) to a certain value in this manner, the circuit can be used as a power divider with the desired power division ratio. Furthermore, the circuit can also be used as a power combiner whereby high frequency signals with a phase difference of 90 degrees are input into ports 2 and 3, and their combined signal is output from port 1. It can also be used as a phase shifter.
Japanese Patent Application Laid Open No. H07-30598 (hereinafter referred to as patent document 1) shows an example of a quadrature modulator comprising a combination of a quadratuer hybrid circuit and a mixer IC. A block diagram of the quadrature modulator described in patent document 1 is shown in FIG. 24. A carrier frequency signal is input into the input port IN of 90 degree phase shifter 190. Said 90 degree phase shifter 190 is comprised of a quadrature hybrid circuit. Outputs OUT1 and OUT2 of 90 degree phase shifter 190, which have a 90 degree phase difference from each other, are multiplied with modulating signals I and Q by multipliers 191 and 192, respectively, to produce modulated carrier waves with a 90 degree phase difference. The output signals of multipliers 191 and 192 are combined by adder 193 and the resulting signal is transmitted to the transmission amplifier circuit, which is not shown in the diagram. In this manner, a quadrature hybrid circuit is used, for instance, in a quadrature modulator, or the like.
Furthermore, Japanese Patent Application Laid Open No. H08-43365 (hereinafter referred to as patent document 2) shows an example of a multiple frequency band phase shifter comprised of multiple quadrature hybrid circuits, each for one of different frequency bands.
Patent document 1 shows in FIG. 25 an example of a quadrature hybrid circuit comprising lumped elements that are equivalent to transmission lines. The transmission line 180 shown in FIG. 23 is replaced with a π type circuit comprised of inductor 194 and capacitors 198 and 199 that are connected to either end of the inductor 194. Likewise, the transmission line 181 is replaced with a π type circuit comprised of inductor 195 and capacitors 199 and 200. The parts that correspond to transmission lines 182 and 183 are the same, so their explanation is omitted.
Here, the capacitors connected on one end to ports 1, 2, 3, 4 have been indicated in abbreviated notation. In brief, two capacitors each need to be connected on one side to each of ports 1, 2, 3, 4 to construct a π type circuit. However, said capacitors are of such capacitance that they are connected between the respective terminals and ground, so they are notated together as a single circuit symbol.
A quadrature hybrid circuit that is equivalent to one with transmission lines can be constructed with π type circuits whose admittance values conform to equations (1) and (2).
As stated in paragraph [0014] of patent document 2, quadrature hybrid circuits have the drawbacks that they can only be used in a limited frequency range, and cannot be used for broad bands. For this reason, multiple quadrature hybrid circuits have conventionally been placed side by side to support multiple frequency bands. Specifically, a configuration with multiple quadrature hybrid circuits, each with all four transmission lines shown in FIG. 23, designed to support a specific frequency band, has been used. Otherwise, when lumped elements are used, there has been a need for multiple quadrature hybrid circuits comprised of inductors and capacitors designed with constants adjusted to each frequency. Therefore, the large size of the resulting circuit has remained a challenge.
In particular, a quadrature hybrid circuit requires a large surface area due to its rectangular shape, as shown in FIG. 23. This is because the transmission lines from each port need to be the same length and space is inevitably wasted in the center of the rectangle. Therefore, multiple use of such circuits necessitates an extremely large circuit surface area.